RFC 3257 SCTP Applicability Statement April 20021 Introduction
SCTP is a reliable transport protocol [RFC2960], which along with TCP
[RFC793], RTP [RFC1889], and UDP [RFC768], provides transport-layer
services for upper layer protocols and services. UDP, RTP, TCP, and
SCTP are currently the IETF standards-track transport-layer
protocols. Each protocol has a domain of applicability and services
it provides, albeit with some overlaps.
By clarifying the situations where the functionality of these
protocols are applicable, this document can guide implementers and
protocol designers in selecting which protocol to use.
Special attention is given to services SCTP provides which would make
a decision to use SCTP the right one.
Major functions provided by SCTP can be found in Appendix A.
1.1 Terminology
The following terms are commonly identified in this work:
Association: SCTP connection between two endpoints.
Transport address: A combination of IP address and SCTP port number.
Upper layer: The user of the SCTP protocol, which may be an
adaptation layer, a session layer protocol, or the user application
directly.
Multihoming: Assigning more than one IP network interface to a single
endpoint.
2 Transport protocols2.1 TCP service model
TCP is a connection-oriented (a.k.a., session-oriented) transport
protocol. This means that it requires both the establishment of a
connection prior to the exchange of application data and a connection
tear-down to release system resources after the completion of data
transfer.
TCP is currently the most widely used connection-oriented transport
protocol for the Internet.
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TCP provides the upper layer with the following transport services:
- data reliability;
- data sequence preservation; and
- flow and congestion control.
2.2 SCTP service model
SCTP is also connection-oriented and provides all the transport
services that TCP provides. Many Internet applications therefore
should find that either TCP or SCTP will meet their transport
requirements. Note, for applications conscious about processing
cost, there might be a difference in processing cost associated with
running SCTP with only a single ordered stream and one address pair
in comparison to running TCP.
However, SCTP has some additional capabilities that TCP lacks and
This can make SCTP a better choice for some applications and
environments:
- multi-streams support:
SCTP supports the delivery of multiple independent user message
streams within a single SCTP association. This capability, when
properly used, can alleviate the so-called head-of-line-blocking
problem caused by the strict sequence delivery constraint imposed to
the user data by TCP.
This can be particularly useful for applications that need to
exchange multiple, logically separate message streams between two
endpoints.
- multi-homing support:
SCTP provides transparent support for communications between two
endpoints of which one or both is multi-homed.
SCTP provides monitoring of the reachability of the addresses on the
remote endpoint and in the case of failure can transparently failover
from the primary address to an alternate address, without upper layer
intervention.
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RFC 3257 SCTP Applicability Statement April 2002
This capability can be used to build redundant paths between two SCTP
endpoints and can be particularly useful for applications that seek
transport-level fault tolerance.
Achieving path redundancy between two SCTP endpoints normally
requires that the two endpoints being equipped with multiple
interfaces assigned with multiple addresses and that routing is
configured appropriately (see Section 3).
- preservation of message boundaries:
SCTP preserves application messages boundaries. This is useful when
the application data is not a continuous byte stream but comes in
logical chunks that the receiver handles separately.
In contrast, TCP offers a reliable data stream that has no indication
of what an application may consider logical chunks of the data.
- unordered reliable message delivery:
SCTP supports the transportation of user messages that have no
application-specified order, yet need guaranteed reliable delivery.
Applications that need to send un-ordered reliable messages or prefer
using their own message sequencing and ordering mechanisms may find
this SCTP capability useful.
2.3 UDP Service model
UDP is connectionless. This means that applications that use UDP do
not need to perform connection establishment or tear-down.
As transport services to its upper layer, UDP provides only:
- best-effort data delivery, and
- preservation of message boundaries.
Applications that do not require a reliable transfer of more than a
packet's worth of data will find UDP adequate. Some transaction-
based applications fall into this category.
3 SCTP Multihoming Issues
SCTP provides transport-layer support for multihoming. Multihoming
has the potential of providing additional robustness against network
failures. In some applications, this may be extremely important, for
example, in signaling transport of PSTN signaling messages [RFC2719].
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RFC 3257 SCTP Applicability Statement April 2002
It should be noted that SCTP multihoming support only deals with
communication between two endpoints of which one or both is assigned
with multiple IP addresses on possibly multiple network interfaces.
It does NOT deal with communication ends that contain multiple
endpoints (i.e., clustered endpoints) that can switch over to an
alternate endpoint in case of failure of the original endpoint.
Generally, for truly fault resilient communication between two end-
points, the multihoming feature needs more than one IP network
interface for each endpoint. The number of paths used is the minimum
of network interfaces used by any of the endpoints. When an endpoint
selects its source address, careful consideration must be taken. If
the same source address is always used, then it is possible that the
endpoint will be subject to the same single point of failure. When
the endpoint chooses a source address, it should always select the
source address of the packet to correspond to the IP address of the
Network interface where the packet will be emitted subject to the
binding address constraint. The binding address constraint is, put
simply, that the endpoint must never choose a source address that is
not part of the association i.e., the peer endpoint must recognize
any source address used as being part of the association.
The availability of the association will benefit greatly from having
multiple addresses bound to the association endpoint when the
endpoint is on a multi-homed host.
4 SCTP Network Address Translators (NAT) issues [RFC2663]
When two endpoints are to setup an SCTP association and one (or both)
of them is behind a NAT (i.e., it does not have any publicly
available network addresses), the endpoint(s) behind the NAT should
consider one of the following options:
(1) When single homed sessions are to be used, no transport addresses
should be sent in the INIT or INIT ACK chunk(Refer to section 3.3 of
RFC2960 for chunk definitions). This will force the endpoint that
receives this initiation message to use the source address in the IP
header as the only destination address for this association. This
method can be used for a NAT, but any multi-homing configuration at
the endpoint that is behind the NAT will not be visible to its peer,
and thus not be taken advantage of. See figure 1.
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RFC 3257 SCTP Applicability Statement April 2002
+-------+ +---------+ *~~~~~~~~~~* +------+
|Host A | | NAT | * Cloud * |Host B|
| 10.2 +--|10.1|2.1 |----|--------------|---------+ 1.2 |
| | | | | * * | |
+-------+ +---------+ *~~~~~~~~~~* +------+
Fig 1: SCTP through NAT without multihoming
For multihoming the NAT must have a public IP address for each
represented internal IP address. The host can preconfigure an IP
address that the NAT can substitute, or, the NAT can have internal
Application Layer Gateway (ALG) which will intelligently translate
the IP addresses in the INIT and INIT ACK chunks. See Figure 2.
If Network Address Port Translation is used with a multihomed SCTP
endpoint, then any port translation must be applied on a per-
association basis such that an SCTP endpoint continues to receive the
same port number for all messages within a given association.
+-------+ +----------+ *~~~~~~~~~~* +------+
|Host A | | NAT | * Cloud * |Host B|
| 10.2 +---+ 10.1|5.2 +-----+ 1.1<+->3.1--+---------+ 1.2 |
| 11.2 +---+ 11.1|6.2 | | +->4.2--+---------+ 2.2 |
| | | | * * | |
+-------+ +----------+ *~~~~~~~~~* +------+
Fig 2: SCTP through NAT with multihoming
(2) Another alternative is to use the hostname feature and DNS to
resolve the addresses. The hostname is included in the INIT of the
association or in the INIT ACK. The hostname must be resolved by DNS
before the association is completely set up. There are special
issues regarding NAT and DNS, refer to RFC2694 for details.
5 Security Considerations
In this section, some relevant security issues found in the
deployment of the connection-oriented transport protocols will be
discussed.
5.1 Security issues with TCP
Some TCP implementations have been known to be vulnerable to blind
denial of service attacks, i.e., attacks that had been executed by an
attacker that could not see most of the traffic to or from the target
host.
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The attacker would send a large number of connection establishment
requests (TCP-SYN packets) to the attacked target, possibly from
faked IP source addresses. The attacked host would reply by sending
SYN-ACK packets and entering SYN-received state, thereby allocating
space for a TCB. At some point the SYN-queue would fill up, (i.e.,
the number of connections waiting to be established would rise to a
limit) and the host under attack would have to start turning down new
connection establishment requests.
TCP implementations with SYN-cookies algorithm [SYN-COOK] reduce the
risk of such blind denial of service attacks. TCP implementations
can switch to using this algorithm in times when their SYN-queues are
filled up while still fully conforming to the TCP specification
[RFC793]. However, use of options such as a window scale [RFC1323],
is not possible, then. With the SYN-cookie mechanism, a TCB is only
created when the client sends back a valid ACK packet to the server,
and the 3-way handshake has thus been successfully completed.
Blind connection forgery is another potential threat to TCP. By
guessing valid sequence numbers, an attacker would be able to forge a
connection. However, with a secure hashsum algorithm, for some of
the current SYN-cookie implementations the likelihood of achieving
this attack is on the order of magnitude of 1 in 2^24, i.e., the
attacker would have to send 2^24 packets before obtaining one forged
connection when SYN-cookies are used.
5.2 Security issues with SCTP
SCTP has been designed with the experiences made with TCP in mind.
To make it hard for blind attackers (i.e., attackers that are not
man-in-the-middle) to inject forged SCTP datagrams into existing
associations, each side of an SCTP association uses a 32 bit value
called "Verification Tag" to ensure that a datagram really belongs to
the existing association. So in addition to a combination of source
and destination transport addresses that belong to an established
association, a valid SCTP datagram must also have the correct tag to
be accepted by the recipient.
Unlike in TCP, usage of cookie in association establishment is made
mandatory in SCTP. For the server, a new association is fully
established after three messages (containing INIT, INIT-ACK, COOKIE-
ECHO chunks) have been exchanged. The cookie is a variable length
parameter that contains all relevant data to initialize the TCB on
the server side, plus a HMAC used to secure it. This HMAC (MD5 as
per [RFC1321] or SHA-1 [SHA1]) is computed over the cookie and a
secret, server-owned key.
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RFC 3257 SCTP Applicability Statement April 2002
As specifically prescribed for SCTP implementations [RFC2960],
additional resources for new associations may only be reserved in
case a valid COOKIE-ECHO chunk is received by a client, and the
computed HMAC for this new cookie matches that contained in the
cookie.
With SCTP the chances of an attacker being able to blindly forge a
connection are even lower than in the case of TCP using SYN-cookies,
since the attacker would have to guess a correct value for the HMAC
contained in the cookie, i.e., lower than 1 in 2^128 which for all
practical purposes is negligible.
It should be noted that SCTP only tries to increase the availability
of a network. SCTP does not contain any protocol mechanisms that are
directly related to user message authentication, integrity and
confidentiality functions. For such features, it depends on the
IPsec protocols and architecture and/or on security features of the
application protocols.
Transport Layer security(TLS)[RFC2246] using SCTP must always use
in-order streams.
Currently the IPSEC working group is investigating the support of
multi-homing by IPSEC protocols. At the present time to use IPSEC,
one must use 2 * N * M security associations if one endpoint uses N
addresses and the other M addresses.
5.3 Security Issues with both TCP and SCTP
It is important to note that neither TCP nor SCTP protect itself from
man-in-the-middle attacks where an established session might be
hijacked (assuming the attacker can see the traffic from and inject
its own packets to either endpoints).
Also, to prevent blind connection/session setup forgery, both TCP
implementations supporting SYN-cookies and SCTP implementations rely
on a server-known, secret key to protect the HMAC data. It must be
ensured that this key is created subject to the recommendations
mentioned in [RFC1750].
Although SCTP has been designed carefully as to avoid some of the
problems that have appeared with TCP, it has as of yet not been
widely deployed. It is therefore possible that new security issues
will be identified that will have to be addressed in further
revisions of [RFC2960].
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RFC 3257 SCTP Applicability Statement April 20029. Full Copyright Statement
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